The present invention relates to a cathode ray tube provided with an electron gun which includes an indirectly heated cathode, and more particularly to a cathode ray tube which enhances the elevation of a cathode temperature of an indirectly heated cathode.
The cathode ray tube which is used in a television receiver set, a display tube and the like possesses the image reproducibility with high definition and hence, the cathode ray tube has been popularly used as a display means applicable to various information processing equipment. This type of cathode ray tube includes an evacuated envelope which is constituted of a panel portion which forms a phosphor screen to which phosphor is coated on an inner surface thereof, a neck portion which accommodates an electrode gun having a plurality of electrodes which are constituted of an indirectly heated cathode, a control electrode and an accelerating electrode for focusing, accelerating and allowing electron beams which are generated by an electron beam generating portion to be irradiated to the phosphor screen, and a funnel portion which contiguously connects the panel portion and the neck portion and exteriorly mounts a deflection yoke for scanning electron beams irradiated from the electron gun on the phosphor screen thereon.
As a heater which is used in the cathode ray tube having the above-mentioned constitution, a heater having the constitution disclosed in Japanese Laid-open 2002-93335 (patent literature 1 U.S. Pat. No. 6,552,479), for example, has been known and one example of such a constitution is shown in FIG. 8 and FIG. 9. Here, FIG. 8 is a side view of the heater and FIG. 9 is an enlarged view of an essential part in FIG. 8.
In FIG. 8 and FIG. 9, numeral 31 indicates a heat-generating core-wire such as a tungsten line, for example, or the like, numeral 32 indicates an alumina insulation film, numeral 33 indicates a dark alumina coated film, numeral 35 indicates a heater, numeral 351 indicates a heat generating portion of the heater 35, numeral 352 indicates a leg portion of the heater 35, numeral 353 indicates a welding portion of the heater 35, and numeral 356 indicates a top portion of the heater 35.
The heater 35 shown in FIG. 8 and FIG. 9 is constituted of the heat generating portion 351 which is formed in a double helical shape by winding the heat-generating core-wire 31 as a coil into the helical structure having a single layer at a given pitch and, thereafter, folding, twisting and winding an approximately center portion of the coil winding portion, straight portions 355 which are formed by winding the heat generating core-wire 31 into the helical structure having a single layer at a pitch equal to the pitch of the heat generating core-wire 31, and leg portions 352 which are contiguously formed with the straight portions 355 and are spirally formed by winding the heat generating core-wire 31 into a coil winding having a plurality of layers such as a five-layered (fivefold) at a pitch coarser than the pitch of the heat generating part 351, and the welding portion 353.
A remaining portion of the coil winding having such a constitution is covered with the alumina insulation film 32 except for the welding portion 353 and, further, the dark alumina coated film 33 containing tungsten fine powder is applied to a surface of the alumina insulation film 32 for performing blackening treatment.
On the other hand, Japanese Patent Laid-open 2002-25422 (patent literature 2) discloses the following constitution. First of all, a primary fine-pitch core-wire coil which leads to a heater main portion is wound at a pitch twice as large as a diameter of the core-wire. Next, the core-wire is folded back and a secondary coarse-pitch core-wire coil having a coarse pitch nine times or more as large as the fine coil pitch is wound thus forming a double winding portion. Further, using the same core-wire, a third fine-pitch core-wire coil is formed at a pitch equal to the pitch of the above-mentioned primary fine pitch, wherein both fine-pitch core-wire coils form single fine winding portions which bite in valleys respectively. Due to the above-mentioned constitution, it is possible to provide an indirectly heated electron tube heater which possesses a large mechanical strength at an end portion thereof, exhibits the low heat generation and can be easily formed.
In this specification, hereinafter, the number (X) of coil winding layers is also referred to as “X-fold winding”, “X-layer winding” or “X-layer structure”.